A thyristor, which is sometimes also referred to as silicon controlled rectifier (SCR), is a switching device which can be turned on when forward biased by a turn-on voltage and when a positive gate current is supplied to a gate terminal. The thyristor is then said to be in a forward conducting state in which a current may flow from an anode to a cathode. On the other hand, the thyristor can also be in a forward blocking state meaning that a high positive voltage can be blocked. In a reverse direction, the thyristor cannot be turned on. A thyristor design may be reverse blocking, which means that it can block approximately the same voltage in the reverse direction as in the forward off-state, or asymmetric, which means that it has virtually no blocking capability in the reverse direction. Since phase control applications commonly require reverse blocking capabilities, a phase control thyristor (PCT) is generally reverse blocking.
For high power applications, thyristors have been developed based on round semiconductor wafers having a diameter of, for example, 4 or 5 inches. However, advanced thyristor applications utilize even larger thyristor designs based, for example, on 6 inch wafers. It has been observed that for such large thyristor designs, it may not be sufficient to simply scale-up previous smaller thyristor designs. With an increasing thyristor diameter, further effects may gain influence on the thyristor operation. For example, a larger thyristor for higher nominal current with equivalent forward blocking capacity or turn-on characteristics as well as cooling characteristics during thyristor operation may not be achieved by proportionally scaling the thyristor dimensions.
DE 1 954 665 discloses a rectifier having a star shaped main gate structure. In between two stripes of the main gate structure, emitter shorts are arranged in the form of a fir tree. There are no emitter shorts arranged in a region close to the main gate structure. Such a fir tree structure gives a coarse structure, which cannot be used for larger devices because the distribution of the branches is too coarse to efficiently influence the blocking capacity and the turn-on characteristics in a way necessary for larger devices.
U.S. Pat. No. 4,903,105 discloses a triac including two thyristors. On the border between these thyristors, a maximum of two emitter short stripes are arranged. The stripes may be formed as a chopped stripe, which have at least 30% of the stripe p doped, that is, the areas between the p doped regions may have a length of less than 70% of the repetition length of the p/n areas of the stripes. The arrangement of these emitter shorts between the thyristors is needed to separate the thyrisors from each other. This is a necessity for the operation of the triac.